A struct where the admin library can store returned performance counters.
Memory regions returned via the struct will be valid until the next call into
the admin library and are managed by the library.  They should not be freed by
the user.

Only one performance counter may be enabled at a time.  Consecutive calls to
\code{enable\_perf\_counters} will return the same ID as the previous call, and
will return performance counters via the previously-used struct.  To change the
structure used for returning performance counters, first disable the performance
counters and then re-enable them.
